1. Field of the Invention
The present invention relates to a microprobe adapted for use as a muscle current measuring probe or in observing apparatus and record/reproducing apparatus such as the probe for a scanning tunnel microscope (STM) or for a force microscope (FM), a method for producing such microprobe, and an information output and/or input apparatus utilizing such microprobe.
2. Related Background Art
A microprobe is often employed in the measurements under a situation with minimized influence to the measured system or in the access to a very small area, namely the measurement of such very small area.
For example, in the neural potential measurement or in the muscle current measurement in a living organism, as the object to be measured is as small as one micron to one millimeter at maximum, the radius of curvature at the pointed end of the measuring probe has also to be likewise one micron to one millimeter. Also the scanning tunnel microscope, which is attracting much attention in recent years as a novel observing method of the resolving power of atomic or molecular size, is considered to attain a higher resolving power as the radius of curvature of the pointed end of the probe, opposed to the specimen surface, is smaller. Ideally said pointed end is sharpened to the atomic size, namely the size of an atom.
Conventionally the probe with such a small radius of curvature at the pointed end is produced by mechanical working or electrolytic polishing. In the mechanical working, a probe with a radius of curvature of 5-10 .mu.m at the pointed end can be obtained by working a fiber-shaped crystal with a watch lathe, and a radius of curvature less than 10 .mu.m can be obtained by die drawing. In the electrolytic polishing, a straight wire-shaped material of a diameter not exceeding 1 mm is immersed vertically by 1-2 mm in electrolyte liquid and is given a voltage intermittently with an interval of 0.2-2.0 seconds under suitable agitation of the electrolyte liquid. This method can provide a probe with the radius of curvature of the order of 0.05 .mu.m at the pointed end.
More recently, the field evaporation method (H. W. Fink, IBM Journal of Research and Development 30, 460, (1986)) is utilized for producing a probe with one to several atoms at the pointed end, with theoretically smallest radius of curvature (R. Allenspach and A. Bischof, Applied Physics Letters 54, 587 (1989)).
The probe with a quite small radius of curvature at the pointed end, produced by the above-mentioned methods, can provide information with an extremely high resolving power when used in observing a specimen of relative smooth surface. However, if the specimen surface has relatively large irregularities as shown in FIG. 1, the probe 1' and the surface of the specimen 2' may be very close or in mutual contact in a position other than the pointed end (longitudinally most extended part) A of the probe, as indicated by an arrow B, whereby the probe may simultaneously pick up information at two or more locations. As the result, the resolving power cannot be reduced to a value matching the radius of curvature at the pointed end of the probe, or the information on a recessed part becomes inaccurate. For example, when the surface of a semiconductor laser grating of a surface structure as shown in FIG. 2 is observed with a scanning tunnel microscope, even a probe with a very small radius of curvature at the pointed end cannot accurately reproduce the surface irregularities of the specimen because of the above-explained reason, and the actual surface irregularities with sharp ridges can only be observed as smoothly modulating irregularities.
In recent years, various memory devices have been very actively developed, as the nucleus of electronic equipment such as computers, computer-related equipment, video disks, digital audio disks etc.
Though dependent on applications, the general requirements for such memory devices can be summarized as:
1) high density and large recording capacity; PA1 2) high response speed in recording and reproduction; PA1 3) low electric power consumption; and PA1 4) high productivity and low cost.
Conventionally the memory devices have been dominated by semiconductor memories and magnetic memories, but, with the advent of laser technology in recent years, there has been introduced the inexpensive high-density optical memories utilizing an organic thin film for example of organic dyes or photopolymers.
On the other hand, the recent development of the scanning tunnel microscope capable of directly observing the electron structure of surfacial atoms of conductive materials [G. Binnig et al., Helvetica Physica Acta, 55, 726 (1982)] enables measurement of the real space image with a high resolving power both in the monocrystalline and in the amorphous materials. Extensive applications are expected for said microscope because it can achieve observation with a low electric power, without damage by current to the observed specimen, and can be used for various materials even in the air.
The scanning tunnel microscope is based on the phenomenon of tunnel current generated when a metal probe electrode and a conductive material are maintained at a very small distance, in the order of 1 nm, with a voltage applied therebetween. Said current is very sensitive to the change in said distance. It is therefore possible to observe the surface structure in real space and to obtain various information on the total electron cloud of surfacial atoms from the amount of vertical movement of the probe, by controlling said distance, with vertical movement of the probe electrode, so as to maintain a constant tunnel current. Also there have been proposed various methods of information recording and reproduction utilizing such scanning tunnel microscope, such as forming a record by modifying the surface state of a recording layer of a suitable recording medium with a particle beam (electron beam or ion beam), a high-energy electromagnetic wave such as X-ray or an energy beam such as visible or ultraviolet light and reading such record with the scanning tunnel microscope, or effecting the recording and the reproduction by the scanning tunnel microscope utilizing a recording layer with a memory effect in the voltage-current switching characteristic, such as a thin film of an organic compound with a .pi.-electron system or a charcogenide compound (as disclosed for example in the Japanese Laid-open Patent Applications 63-161552, 63-161553 and 63-204531).
FIG. 3 shows an example of the probe electrode to be employed in such record/reproducing methods.
FIG. 3 shows a state in which a probe electrode 81 is positioned close to a recording medium 82 with surface irregularities. The resolving power of such probe electrode 81 is generally higher as the radius of curvature at the pointed end becomes smaller.
The probe electrode of small radius of curvature at the pointed end, employed for the above-mentioned purposes, is conventionally produced by mechanical working or electrolytic polishing, like the probes for the aforementioned observing apparatus.
Also the aforementioned field evaporation method is recently utilized for producing a probe electrode with one to several atoms at the pointed end, corresponding to the theoretically smallest radius of curvature.
The probe electrode with such small radius of curvature enables recording and reproduction in the atomic order, namely several Angstroms.
The probe electrode with a very small radius of curvature at the pointed end, obtained by the above-mentioned conventional methods, can exactly record the information with an extremely high density, if the recording medium has a completely flat surface. In practice, however, such flat recording medium is difficult to obtain, and the surface of the medium often shows significant irregularities. On such medium, the conventional probe electrode 81 may contact or may be positioned very close to the surface of the recording medium 82 in a position other than the longitudinal end of the probe electrode, as indicated by an arrow in FIG. 3, whereby the probe electrode 81 will record the information in an erroneous position of the recording medium 82. Consequently, the information recording with such probe electrode with result in drawbacks that the recorded information cannot be reproduced or that a recording density matching the radius curvature of the pointed end of the probe cannot be obtained. Thus the recording yield has been very poor as the surface of the recording medium has been utilized only in a very limited smooth area. Also in case of a recording medium with tracking grooves as shown in FIG. 12, with the reduction in pitch of the grooves for elevating the recording density, the record bits become more difficult to form in recesses portions which are more resistant to destruction for example by abrasion, and have to be formed on the protruding portions which are more apt to be destructed by abrasion.